Magnetic N-carboxymethy chitosan as a kind of potential magnetic resonance imaging contrast agent was synthesized by a simple and economic route (compared with traditional Microemulsion and Thermal Decomposition). N-carboxymethyl chitosan was synthesized by the process of carboxymethylation of chitosan firstly. Then, Fe3O4 nanoparticles were obtained by in-situ reaction in the chains of N-carboxymethyl chitosan. The characterization and performance of magnetic N-carboxymethy chitosan were tested. The result of TGA showed that the generation amounts of Fe3O4 were related to the contents of catboxymethyl groups in N-carboxymethy chitosan. It increased unceasingly along with the increase of the contents of catboxymethyl groups. But the generation amounts of Fe3O4 reach a peak when the contents of catboxymethyl groups increased to some degree. The result of TEM presented that the average grain size of the Fe3O4 nanoparticles was about 5-10 nm. The MRI depicted that the magnetic N-carboxymethy chitosan provided high transversal relaxation rate with 82.82 mmoL/L/s. It was higher than the minimum standard of R2 being over 62 mmoL/L/s when superparamagnetic iron oxide as magnetic resonance imaging contrast agent. The magnetic N-carboxymethy chitosan could be used as a kind of potential magnetic resonance imaging negative contrast agent.